The invention relates to a process for the preparation of a stabilized organophosphorous composition.
Organophosphites and polyorganophosphites have been used for a variety of applications including preservatives (e.g., antioxidants) for plastic materials and as ligands for catalysts. However, phosphite ligands may decompose during long-term storage, primarily due to hydrolysis of the phosphite moiety, which produces acidic by-products that may catalyze additional degradation. Even phosphite products manufactured in high purity and packaged under stringent conditions may slowly degrade over time. Significant decomposition may require expensive reprocessing to remove the acidic by-products prior to industrial use, e.g. in a hydroformylation process, to avoid accelerated hydrolysis and/or solids formation within the production unit.
The initial acid content of the organophosphite is particularly important if the ligand is to be introduced to the end use process slowly as a solution over time, which is a common practice when it is desired to maintain the ligand concentration at a constant level. If the solid phosphite contains appreciable amounts of acid, dissolution can result in rapid hydrolysis that will consume a significant portion of the valuable ligand within days.
U.S. Pat. No. 4,835,299 discloses a method for purifying tertiary organophosphites during manufacture. Secondary organophosphite impurities are converted to primary organophosphite salts by treatment with aqueous base, and the desired tertiary organophosphite is recovered via crystallization. That process may be appropriate for reprocessing of partially decomposed ligand, but it is not suitable for routine use in a hydroformylation facility, and doesn't address storage of ligand solutions.
US 2013/0225849 discloses the use of trace amounts of sodium methoxide as an additive in a washing step during the purification phase of the ligand manufacturing process to address the instability of polyorganophosphites in the presence of residual solvent. However, the presence of such an extremely strong base is not suitable in many catalytic processes, such as hydroformylation, hydrocyanation or hydrogenation. US 2013/0225849 also teaches that the strong base is removed prior to packaging.
WO 2012/145241 teaches how to store active transition metal-ligand catalyst solutions by reducing water or acid content prior to storage using an extractor.
U.S. Pat. No. 8,461,394 describes the use of hindered amines, e.g. piperidines, to stabilize bisphosphite-promoted hydroformylation processes. For example, the bisphosphite and amine are combined for daily additions. Insoluble salts are formed and filtered away. Employing such a process with ligand that is already significantly degraded would generate large amounts of insoluble salts that could plug lines and degrade pumps. The addition of the amine/ligand solution to the hydroformylation reaction system is immediate, and no teaching about the long term stability of such a mixture is presented.
Hydrolyzable organophosphorus ligands employed for hydroformylation must be added to the process at a rate comparable to their degradation rate, as taught in, e.g., U.S. Pat. Nos. 5,741,945, 5,741,943 and 7,863,487.
The organophosphorous ligand may be added in a batch manner; for example, the solid ligand may be added to a separate vessel, e.g., a mix tank, dissolved in product aldehyde and added to the process within a single day. Even over a short period of time, hydrolytic degradation can occur if the organophosphorous compound added to the mix tank contains significant levels of acid. Hydrolysis can be exacerbated by high levels of water or carboxylic acids, which can form in aldehydes upon exposure to air. Because hydrolysis results in acid by-products, an autocatalytic scenario could potentially develop wherein the bulk of the expensive ligand is consumed before it ever reaches the hydroformylation process.
In other cases, it is advantageous to add the ligand to the process on a continuous basis. In one method, the ligand and aldehyde product are combined in the mix tank, and the resulting solution metered into the system at an appropriate rate. Effective implementation of a continuous addition strategy requires storage of the ligand solution in the mix tank for extended periods, e.g., days, weeks or even longer. Stability of the solution is impacted by both the acid content and the extended time in storage, thus a constant ligand addition strategy presents a greater risk for excessive ligand degradation.
Accordingly, there is a need for an easy, cost-effective means to utilize hydrolyzable organophosphorus ligands, which have partially degraded during storage, within a hydroformylation process, and to add that ligand as a solution over time without excessive degradation of the valuable ligand compound.